Electromagnetic actuators as used, for example, for operating cylinder valves in piston-type internal-combustion engines, have at least one electromagnet which exerts an electromagnetic force on an armature supported by resetting means and moving a setting member. The energized holding magnet holds the armature in one of its operational positions so that by de-energizing the holding magnet the armature is moved into its other operational position by the resetting means.
In actuators which are used for operating cylinder valves constituting the setting members, the course of the current control has a significant influence on the various parameters, for example, the condition of the combustion mixture in the intake zone, the condition in the combustion chamber as well as in the exhaust region and furthermore significantly affects the combustion process itself in the combustion chamber. Since the internal-combustion engines operate in a non-stationary manner in widely varying operational conditions, a correspondingly variable and adaptable control of the cylinder valves is necessary.
A significant problem in the control of an electromagnetic setting device of the above-outlined type resides in the required timing accuracy and, connected therewith, in the energy consumption as it is particularly required in case of a load-dependent engine-output control effected by the intake valve actuation. A precise control of the timing is made more difficult because of manufacturing tolerances, wear phenomena occurring during operation, and various operational conditions, such as changing load requirements and working frequencies since these external influences affect time-relevant parameters of the system.
A significant problem in an electromagnetic setting device of the above-outlined type is the sticking of the armature to the momentary holding magnet after the de-energization of the magnet coil. Such sticking is caused essentially by the eddy currents in the magnetic circuit. It has been attempted to configure the magnet yoke and the armature such that the generation of eddy currents is substantially eliminated or the disadvantageous effect of such eddy currents is compensated for. The sticking period depends from numerous various parameters such as the size of the air gap between the pole face and the armature situated at the holding magnet, the force of the resetting means (which, as a rule, are mechanical springs), and accelerations affecting the setting unit. Apart from the unavoidable manufacturing tolerances, the alternating gas counter pressures affecting the cylinder valves and the acceleration forces which have an unpredictable magnitude and which act on the armature, cause irregular oscillations of the sticking period, so that after switching off the holding current, the motion start of the armature varies in an undeterminable manner. Also, the duration of motion (flying time) of the armature as well as the kinetic energy losses and thus the electric energy to be applied to the opposite (capturing) magnet depends from the momentary operational condition.
German Offenlegungsschrift (application published without examination) 195 26 681 describes a method for a timely accurate control of the armature motion of an electromagnetically operated setting arrangement of the above type in which after the lapse of a predetermined period following the switch-off of the holding current, a short current pulse of inverted polarity is applied to the magnet coil. Such a short current pulse in which the current intensity, the duration and the moment of energization are appropriately selected, may result, to be sure, in a faster decay of the magnetic field generated by the eddy currents in the magnet yoke of the electromagnet, and thus the armature is released from the pole face of the holding electromagnet in a timely accurate and reliable manner. With such an arrangement constant sticking periods for the control may be achieved. Since the eddy currents cause, particularly in the armature upon its release from the pole face, a buildup of a magnetic field which opposes the force of the resetting means affecting the armature and which delays the armature motion, the earlier-noted kinetic energy loss is eliminated. Such a loss can be compensated for only by an increased amount of electric energy applied to the capturing electromagnet if the actuator is to operate reliably and in a timely accurate manner. The short current pulse for causing a decay of the magnetic field in the electromagnet is not sufficient for such a purpose. Such disadvantages may be reduced only by an armature structured to suppress eddy currents. The residual magnetization of the magnet yoke and the armature following the switch-off of the current causes a significant energy loss which, at the capturing magnet, must be compensated for by a corresponding increase of energy input which, however, cannot be applied with the known process as an additional short current pulse.